The present invention generally relates to optical writing apparatus, and more particularly to a multipoint synchronization optical writing apparatus which is applied to high quality laser beam printers and laser plotters.
Generally, in an optical writing apparatus of, for example, a laser beam printer, a writing beam emitted from a laser light source is modulated in accordance with image information, and this writing beam is deflected by a deflector unit, for example, a rotary polygonal mirror, to scan the deflected light over a surface of a photosensitive body where an optical writing of the image information is carried out. However, it is quite difficult to rotate the rotary polygonal mirror at a substantially constant speed, and there are usually variations of rotating speed when the rotary polygonal mirror is driven. Actually, such speed variations lead to irregular pitches of scanning dots on the surface of the photosensitive body. For achieving good positioning accuracy of optical writing dots with regular pitches of the scanning dots, U.S. Pat. No. 3,389,403 discloses a conventional optical writing apparatus of a multipoint synchronization type which uses a linear encoder having a plurality of slits for supplying a multipoint synchronizing signal to control the modulation of the writing beam. But it is difficult to manufacture such a conventional linear encoder product having a greater number of slits for providing a very fine resolution.
There is also a conventional multipoint synchronization optical writing apparatus, as disclosed in Japanese Published Patent Application No. 54-97050, which discloses some improvements of the above discussed optical writing apparatus, one of the improvements being to facilitate the manufacture of the linear encoder that provides a finer resolution. In this conventional multipoint synchronization optical writing apparatus, a video clock, or a picture element clock signal, is produced by dividing a pulse repetition rate of a photoelectrically converted signal, sent from the linear encoder, by n. More specifically, a phase detector (PD) of a phase lock loop (PLL) circuit compares the photoelectrically converted signal obtained from the linear encoder with a predetermined reference signal, and a feedback control of a phase of a signal from a voltage controlled oscillator is carried out so that both the phases of these signals accord with each other. The PLL circuit provides the laser light source with a synchronizing signal for controlling the modulation of the laser light source, the synchronizing signal being in synchronism with the photoelectrically converted signal and obtained by dividing the pulse repetition rate of the photoelectrically converted signal by n. The PLL circuit usually shows a certain degree of response time delay owing to the circuit structure. Immediately after the photoelectrically converted signal is generated, the phase of the photoelectrically converted signal normally does not accord with the phase of the reference signal, and this time period required for phase coincidence is called a lockup time. In the above discussed conventional optical writing apparatus, after the lockup time of the PLL circuit elapses and the phases of the signals accord with each other, a gate circuit is turned ON to allow the PLL circuit to first output a write set signal that starts to send a picture element clock signal. A well controlled modulation of the writing beam from the laser light source in synchronism with this picture element clock signal allows each starting point of effective scanning lines on the photosensitive body to be aligned accurately in a vertical scanning direction, ensuring a desired accuracy for writing dot positions. In general, the shorter the above discussed lockup time of the PLL circuit is, the longer the effective scanning line is. A longer effective scanning line is advantageous for obtaining a fine resolution from an optical writing apparatus. In this regard, the above discussed optical writing apparatus uses the first pulse of the photoelectrically converted signal to reset a divider for dividing of the pulse repetition rate of the photoelectrically converted signal to a turned-off state. This resetting of the divider causes the phase of the photoelectrically converted signal to accord with that of the reference signal, thereby reducing the amount of phase compensation and shortening the lockup time of the PLL circuit.
However, it is only within a limited range of ambient temperature that the above discussed conventional optical writing apparatus can accurately position writing dots on the photosensitive body. Once the ambient temperature changes greatly, the PLL circuit often shows, even when the phase is locked, a great change in phase difference between the reference signal and the clock signal. The clock signal is provided by dividing the pulse repetition rate of the output signal supplied from the PLL circuit as a feedback signal. Therefore, even if a phase difference between the write set signal (in synchronism with the reference signal) and the writing clock signal (the picture element clock signal) at a normal temperature is correctly adjusted, the phase difference between the write set signal and the write clock signal is increased when the ambient temperature changes significantly. This may result in writing dots being placed at undesired positions of an image forming surface. In other words, in the case of the conventional optical writing apparatus, there is no effective measure for compensating for such a change in phase difference when the ambient temperature changes.
In addition, since the reference signal occurs intermittently, an input voltage to the voltage controlled oscillator fluctuates when such an intermittent reference signal occurs. This makes a ripple of the input voltage become large, and there is an undesired response delay of the PLL circuit to function in step with the synchronization.